Conventionally, there has been known an optical recording apparatus wherein a recording medium is heated by using energy of a laser light beam to change magneto-optical characteristics thereof to thereby record data thereon.
At first, a conventional optical recording apparatus will be explained with reference to FIGS. 1 and 2.
An example of the construction of the conventional optical recording apparatus is shown in FIG. 1. For simplifying the explanation, the explanation of various types of servo control circuits will be omitted in this conventional example of FIG. 1.
Referring now to FIG. 1, reference numeral 1 denotes a magneto-optical disc acting as an optical disc which is capable of rewriting data and rotated at a constant angular velocity or a constant linear velocity by a spindle motor 2. Reference numeral 10 denotes an optical head having a laser diode 11 and a photo diode 12 for writing data to (or reading data from) the magneto-optical disc 1. A magnetic head 10a acting as an external magnetic field supplying apparatus is disposed opposite the optical head 10 through the magneto-optical disc 1 in a manner that the magnetic head and the optical head sandwich the magneto-optical disc 1 in an opposite manner.
In this construction, the magnetizing direction of a part of a vertical magnetic recording film of the magneto-optical disc 1 where a laser light beam from the optical head 10 is radiated is changed in accordance with a direction of external magnetic field supplied from the magnetic head 10a.
Reference numeral 20 denotes a recording circuit system, wherein digital data as an information signal to be recorded on the magneto-optical disc 1 is supplied to an encoder 21 through an input terminal IN and converted not only into a predetermined format but also into a recording signal formatted with a modulation method such as a pulse position modulation, for example, by the encoder 21. The output of the encoder 21 is supplied to a light intensity modulation circuit 22 which in turn delivers an output signal to the laser diode 11 through a driving amplifier 23 to thereby intermittently control the intensity of radiated light from the diode.
A part of the laser light beam radiated from the laser diode 11 is reflected by a prism mirror and detected by the photo diode 12. The detected output of the photo diode 12 is supplied to a comparator 25 through an amplifier 24 and compared therein with a reference value applied from a reference value setting circuit 26. The output from the comparator 25 is fed back to the light intensity modulation circuit 22 to control the light intensity (power level) of the laser diode 11 at a constant value, thereby performing a so-called automatic power control (APC).
Now, with reference to FIG. 2, a method of forming a recording area (mark) in the conventional optical recording apparatus will be explained.
When the recording linear velocity of the magneto-optical disc 1 is 10 m/s, for example, a laser light with a power level of 10 mW and a pulse width of 50 nS, for example, is radiated from the diode 11 as shown in FIG. 2A, the temperature of a recording layer of the magneto-optical disc 1 increases and then decreases as shown in FIG. 2B, so that a mark with a length twice as large as the pulse width of the radiated laser beam, that is, a time length of 100 nS is recorded in the recording layer with a Curie point Tc of 180.degree. C., for example, illustration thereof being omitted. In this case, the temperature of the recording layer corresponds to the center of radiation of the laser light beam whose energy density is accorded with Gausian distribution and is influenced also by thermal diffusion in the recording layer.
In the above-described example, as clear from FIG. 2B, at a time where a time period corresponding to a beam radiation length twice as large as the mark length, that is, 200 nS has lapsed after initial start of the radiation of the laser light beam, the temperature of the recording layer of the magneto-optical disc 1 has decreased to a value almost same as that before the beam radiation thereon, so that a succeeding mark can be formed without being influenced by the remaining heat generated when forming the preceding mark. Thus, the modulation method such as the pulse position modulation can be employed without any difficulty.
By the way, in order to perform a high-density recording, it has been proposed to set a recording linear velocity same as that of the above-described example and set a time interval of radiation of the laser light beam half of that of the above-described example, for example, that is 25 nS, thereby forming a mark with a time length of twice the radiation time interval, that is, 50 nS.
In this case, as known by JP-A-58-212628 filed by the same applicant as the present application, for example, due to the influence by the thermal diffusion of the recording layer, it is required to reduce a pulse width of the radiated laser light beam and to increase a power level thereof when compared with the above-described example. Namely, a laser light with a pulse width of 15 nS and a power level of 20 mW, for example, is radiated from the diode 11 as shown in FIG. 3A to thereby increase and then decrease a temperature of the recording layer of the magneto-optical disc 1 as shown in FIG. 3B to form a mark with a predetermined time length, that is, 50 nS.
In this case, however, as clear from FIG. 3B, at a time where a time period corresponding to a beam radiation length twice as large as the mark length, that is, 100 nS has lapsed after initial start of the radiation of the laser light beam, the time period lapsed after termination of the laser beam radiation is such a shorter value, as almost half of that of the above-described example, so that a temperature of the recording layer of the magneto-optical disc 1 merely decreases to a value higher by about 40.degree. C. than a temperature at the initiation of the laser beam radiation.
This residual temperature increase, that is, remaining heat generated when forming the preceding mark is added to the temperature increase of the recording layer when forming a succeeding mark as shown by a dotted line in FIG. 3B, so that a time period required to reach to Curie point Tc in the recording layer is made shorter in a temperature rising mode and a time period required to reach to Curie point Tc in the recording layer is made longer in a temperature falling mode. Thus, front and rear edges of a mark to be formed are shifted to a front and a rear side from predetermined positions respectively and so a desired mark can not be formed accurately, whereby there was such a problem that an error occurs in data reproduced from the marks in the modulation method of the pulse position modulation type.